Ultrasonic imaging systems hold the promise of picturing the inner workings of a particular medium such as a human body without invasive surgical procedures, etc. Generally, such systems generate ultrasonic pulses that propagate through the body or other medium and the various structures therein generate echoes that are detected by the ultrasonic system. These echoes are used to generate an image of the interior structure of the medium.
In some cases, the medium may exhibit a nonlinear response to the ultrasonic pulses. At times, it is desirable to isolate the non-linearity in the medium to generate images of the medium in question. Among the techniques that have been developed to isolate the non-linearities is harmonic imaging. In harmonic imaging, a narrowband ultrasonic pulse is transmitted into the medium and the echo signals received are filtered to isolate the second harmonic that is believed to contain primarily non-linear echo information. However, this approach provides poor spatial resolution because it is a narrowband technique.
Another approach is to use phase inversion techniques to cancel out linearities. According to this approach, a relatively wideband, ultrasonic pulse is first transmitted into the medium. Shortly thereafter, the same pulse with an inverted polarity is transmitted as well. The received echo signals are added to each other, and presumably any linearities are canceled. Unfortunately, this approach suffers due to its susceptibility to motion artifact that results in unsatisfactory imaging.